Auto-gain correction and common mode voltage cancellation in a precision amplifier

ABSTRACT

Auto-gain correction in a precision amplifier provides continuous calibration of the gain of the two differential input stages relative to each other and thus significantly minimizes the effects of device mismatch and temperature. Auto-gain correction together with auto-zero minimizes the effects of common mode input voltage on the amplifier and eliminates the need for trim associated with the matching of the two differential input stages. Improved gain matching enhances the accuracy of the auto-zero, which further improves the accuracy of auto-gain correction, resulting in a synergy with both operating together. The implementation of the auto-zero using an input pair of series capacitors in conjunction with a common input reference and a feedback pair of series capacitors in conjunction with a common feedback reference provides for decoupling the common mode voltage of the input differential pair or feedback differential pair. Various features may be used in sub-combinations as desired.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of precision instrumentationamplifiers that provide high gain accuracy and low input offset.

2. Prior Art

The accuracy of the DC output of a linear amplifier is subject to thephysical imperfections of the individual devices comprising theamplifier. The errors caused by the imperfections are Input-Offset (VOS)and Gain Error (GE). The amplifier output (VO) is the algebraic sum ofthe amplifier Input (VI) and Input-Offset times the amplifier Gain (GA).The Input-Offset of the amplifier is the input required for a zerooutput. In practice the amplifier input offset is determined from theoutput of the amplifier for a zero input divided by the gain of theamplifier.VO=GA*(VOS+VI)VO=GA*VOS for VI=0or VOS=VO/GA for VI=0

Generally, an instrumentation amplifier has two differential inputstages with low input bias currents. FIG. 1 shows one form of aninstrumentation amplifier with differential inputs INP and INN, andoutput VO with output reference VREF, and with 2 gain setting resistorsR1 and R2. The first differential input stage (GM1) is the amplifierinput path (INP & INN), the second differential input stage (GM2) isfunctionally identical to GM1 and is the feedback path which takes afraction of the amplifier output (VFB−VREF) and feeds it back. These twodifferential inputs are then subtracted to determine an error. Theoutput stage (GM3) adjusts the output voltage VO to try to force theerror to zero, which results in the amplifier differential input and thefeedback differential input being equal. If GM1 and GM2 are identicaland the feedback error is zero, the ratio of the output to the feedbackinput is equal to the ratio of the total resistance to the feedbackresistance, which is the gain of the instrumentation amplifier.

In an integrated circuit, input offsets are a result of minor defectsand minor device mismatches in amplifier gain stages. Methods forobtaining extremely low input offsets have been well understood and usedfor many years prior to the development of monolithic integratedcircuits. These methods usually involve some form of or combination ofchopper amplifier, chopper stabilization and auto-zero. These techniquesare very amenable to semiconductor fabrication, are effective over avery wide temperature range, eliminate the need for offset trimming andlower the overall cost of production.

Conversely, methods to obtain gain accuracy have usually required theneed for trimming, and usually do not obtain gain accuracy over a widetemperature range or a wide input common mode voltage range, where theinput common mode voltage is the average of the two differential inputsrelative to a common ground.

The lack of gain accuracy is referred to as Gain-Error (GE). One sourceof Gain Error involves the accuracy of the feedback. A resistor dividernetwork in the output path usually determines the feedback differentialinput. The relative accuracy of this resistor divider ratio limits thegain accuracy of the amplifier. Semiconductor techniques of resistormatching and resistor trimming allow for very accurate control ofresistor divider ratios over wide temperature ranges, which in turn arevery effective in minimizing the adverse impact of resistor dividerratios on gain accuracy.

The other source of gain error is the relative lack of matching of thetwo differential input stages of the instrumentation amplifier. Thesetwo input stages take the differential voltage at their inputs and eachcreates an output that will be differenced with the output of the otherinput stage to generate an error signal. The outputs of these inputstages may be a current or a voltage, but in either case if these inputstages are not identically matched, there will be a resulting gainerror. Here again, trimming has been used to improve the matching of thetwo differential input stages. There are severe limitations to theeffectiveness of trimming because the source of mismatch is not fromresistors but from active semiconductor devices operating over widelydiffering common mode input voltage conditions and over wide temperatureranges.

FIG. 2 is an example of a chopper-stabilized instrumentation amplifier.The upper circuit path is a continuous, high-speed instrumentationamplifier, and constitutes the main amplifier. Continuous here impliesthat the amplifier is a linear, continuous time architecture and thatthere is no interruption to the signal flow. The circles at thenon-inverting inputs with the label VOS represent the fact that theamplifiers are not perfect and have a finite input offset. Only oneinput offset VOS is shown in each amplifier, even though the amplifiermay have two parallel input stages, the VOS shown being the netdifference between the two differential inputs. The lower circuit pathis the chopper correction path. The blocks with the X represent thechoppers that are simply series switches which during one half thechopper period are connected from the input straight across to theoutput, and during the other half of the chopper period are connecteddiagonally across. The amplifier between the chopper blocks is aninstrumentation amplifier that outputs a differential currentproportional to the difference between the two pairs of differentialinput voltages. The amplifier block with the capacitors connected fromoutput to input is used as an integrator, and integrates thedifferential current from the chopped instrumentation amplifier output.The output of the integrator is a differential voltage and the amplifierstage to the right of the integrator is a simple V to I (i.e., voltageto current or transconductance) conversion stage which outputs adifferential current proportional to the differential voltage at theintegrator output. This differential current is the correction currentwhich is applied to the main amplifier.

The main amplifier is high bandwidth and dominates the amplifier outputat high frequencies while the correction current from the chopper pathdominates at low frequencies due to the error integrator in the chopperpath. Offsets are extremely low frequency effects and are thus minimizedby the chopper path.

The chopper-stabilized amplifier is well established in practice, and inprofessional and patent literature. For more recent developments, seefor instance, U.S. Pat. No. 7,132,883, entitled “ChopperChopper-Stabilized Instrumentation and Operational Amplifiers”, and U.S.Pat. No. 7,209,000, entitled “Frequency Stabilization ofChopper-Stabilized Amplifiers”, both of which are assigned to MaximIntegrated Products, Inc., the assignee of the present invention, thedisclosures of which are hereby incorporated by reference. In general,the chopper frequency selected will be dependent on many parameters inthe design and application. A typical frequency might be 10 kHz to 50kHz.

The imperfections of the instrumentation amplifier between choppersresulting in input offset errors are represented by the circles with VOSat the non-inverting inputs. The result of proper application ofchopping is to reduce the effect of the input offsets by several ordersof magnitude. Thus the chopper path is the low offset path and theoutput of the chopped instrumentation amplifier is integrated (i.e.,error is continuously accumulated) providing correction to the maininstrumentation amplifier. Once the two differential inputs of thechopped instrumentation amplifier are equal the differential outputcurrent is zero and the integrator maintains that value as thecorrection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one form of a prior art instrumentation amplifierwith differential inputs INP and INN and output VO with output referenceVREF.

FIG. 2 illustrates another form of a prior art instrumentation amplifierusing chopper stabilization.

FIG. 3 illustrates an exemplary auto-zero circuit with input andfeedback common-mode voltage cancellation used in the present invention.

FIG. 4 illustrates an exemplary gain correction circuit used in thegain-corrected instrumentation amplifier of FIGS. 6 a, 7 a, and 8 a.

FIG. 5 a illustrates the features of both FIG. 3 and FIG. 4 in aninstrumentation amplifier.

FIG. 5 b illustrates the features of both FIG. 3 and FIG. 4 in aninstrumentation amplifier with choppers.

FIG. 6 a is a diagram illustrating a gain-corrected feedforwardinstrumentation amplifier system using the gain correction circuit ofFIG. 4.

FIG. 6 b is a diagram illustrating a gain-corrected, auto-zeroedfeedforward instrumentation amplifier system using the combinedauto-zero correction and gain correction configuration ofinstrumentation amplifier of FIG. 5 a.

FIG. 7 a is a diagram illustrating a gain-corrected chopper-stabilizedfeedforward instrumentation amplifier system using the gain correctioncircuit of FIG. 4.

FIG. 7 b is a diagram illustrating a gain-corrected, auto-zeroed,chopper-stabilized feedforward instrumentation amplifier system usingthe combined auto-zero correction and gain correction configuration ofinstrumentation amplifier of FIG. 5 a.

FIG. 8 a is a diagram illustrating a gain-corrected, chopper-stabilizedfeedforward instrumentation amplifier system with sample-and-hold usingthe gain correction of FIG. 4.

FIG. 8 b is a diagram illustrating a gain-corrected, auto-zeroed,chopper-stabilized feedforward instrumentation amplifier system withsample-and-hold using the combined auto-zero correction and gaincorrection configuration of instrumentation amplifier of FIG. 5 a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a means for continually calibrating thegain of the two differential input stages relative to each other andthus significantly minimizes the effects of semiconductor devicemismatch, temperature and common mode input voltage. Embodiments of thisinvention also make use of auto-zero techniques in conjunction withchopper-stabilized techniques to eliminate trim and effectively reducegain error associated with the matching of the two differential inputstages.

FIG. 3 illustrates the use of auto-zero techniques with input andfeedback common mode voltage cancellation on the instrumentationamplifier between the choppers of FIG. 2. In FIG. 3, it is useful toillustrate the instrumentation amplifier as two distincttransconductance stages with outputs differenced (subtracted from oneanother). The objective of auto-zeroing is to effectively remove theoffset of an amplifier by inserting series capacitors (CAZ) with theinverted offset voltage stored thereon. At some regular interval, thesignal path is interrupted and the voltage stored on the capacitors areupdated. The voltages are updated by opening the input and outputswitches (SWI), disconnecting the input, output and feedback signals,and by closing the auto-zero switches (SWAZ) at the combined outputs ofthe two amplifier stages. In the embodiment shown in FIG. 3, all fourinputs are capacitor coupled by capacitors CAZ, with an additional setof switches (SWAZ) that tie the input side of the CAZ capacitors foreach amplifier in common to a common mode reference voltage: Input VCMRef for the input pair of capacitors, and Feedback VCM Ref for thefeedback set. This use of the Input VCM Ref and the Feedback VCM Ref forthe common mode voltage cancellation embodies one of the features ofthis invention. This connection ties the amplifier in a unity gainconfiguration. There are various topologies which can be used forauto-zero that vary from a single CAZ capacitor storing the total inputoffset for both input stages of the amplifier, all the way to a topologysimilar to that shown with four CAZ capacitors, where the total offsetvoltage of the two stages is split equally between all CAZ capacitors.The particular topology shown has the advantage that the common modevoltage in both the input and the feedback may be effectively removed ifInput VCM Ref is equal to the input common mode and Feedback VCM Ref isequal to the output common mode. Assuming a high gain, the differentialinputs will be small, so this may be approximated by letting Input VCMRef be equal to one of the input signals (INP or INN) and Feedback VCMRef be equal to one of the feedback signals (INP_FB or INN_FB).

The incentive to use auto-zero techniques in addition to chopping,instead of chopping only, is to reduce output ripple. When chopping isused without auto-zero, the result of chopping will be to create atriangle wave at the output of the chopper-stabilized system. The outputwill be effectively ramping toward a positive output offset, then towarda negative output offset, and the magnitude of the triangle wave will bedependent on the magnitude of the offset, the chopping frequency and thebandwidth of the chopper path. Auto-zero without chopping will reduceripple but will not reduce offset to as low a level as will chopping.Together, offset and ripple are both minimized.

When auto-zero is added to a chopper-stabilized amplifier system, thefrequency of auto-zero and the duration of the auto-zero connection arenot predetermined by the chopping frequency. The frequency and thefraction of time spent in auto-zero are design variables available tooptimize a design. Even with the benefits of auto-zero there is anincentive to reduce the percent of time spent in auto-zero since that iswhen the useful input signal is blocked, and increasing the fraction oftime spent in auto-zero can negatively impact bandwidth and phase margin(amplifier stability).

FIG. 4 shows an instrumentation amplifier similar to that of FIG. 3 withconfiguration switches SWGM at the inputs and outputs. This Figureillustrates the configuration needed for the gain correction whichembodies one of the features of this invention. The two stages GM4 andGM5 from FIG. 3 are shown, and each has the circle with VOS (typicallydifferent offsets) at the non-inverting input. There is a thirdamplifier stage between GM4 and GM5 that will be called the gaincorrection amplifier stage. In this instance, the two stages GM4 and GM5are now variable transconductance amplifiers.

Transconductance is the gain associated with converting from adifferential voltage to a differential current. A simpletransconductance stage is a differential pair of MOS devices with acurrent source connected to the common source of the differential pair.A variable transconductance amplifier is one in which the gainassociated with converting differential voltage to differential currentis made variable. One way of implementing this would be to make thecurrent source connected to the common source of the differential paircontrollable. The transconductance would increase as the magnitude ofthe current is increased. In general, transconductance amplifiers arewell known in the art. Variable transconductance amplifiers are a subsetof Variable Gain Amplifiers (VGA) and a discussion using CMOStransistors can be found in: Behzad Razavi, Design of Analog CMOSIntegrated Circuits, First Ed., New York: McGraw-Hill, 2001.

FIG. 4 takes into account another imperfection in the manufacture ofintegrated circuits in addition to input offset discussed with respectto FIG. 3, that being that the two gain stages GM4 and GM5 are notperfectly matched. This is represented by GM5=GM4+ΔGM, where ΔGMrepresents the GM error between the two GM stages.

In FIG. 4 the CAZ capacitors and the SWI and SWAZ switches from FIG. 3are not shown. Instead there are SWGM switches which, as shown,disconnect the input, output and feedback signals from theinstrumentation amplifier, and connect a differential reference voltageΔVT=[VT+]−[VT−] to each of the inputs. This is called the GM correctionconfiguration. Assume for the moment that the auto-zero cycle has justtaken place and the proper offset compensating voltage is held on theCAZ capacitors (not shown in FIG. 4). The differential reference voltage[VT+]−[VT−] will be appropriately fixed within the expected range of theamplifier input signals, such as somewhere between 20 mV and 100 mV forexample. The outputs of the instrumentation amplifier are also switchedvia the SWGM switches to the differential inputs of the gain correctionamplifier GM6, with a capacitor CGM across the differential input to GM6to store the voltage when these switches are opened. The objective ofthe GM correction configuration is to force the GM error (ΔGM) betweenthe two GM stages of the instrumentation amplifier to a minimum. Likethe auto-zero connection, this connection is made periodically for abrief moment. When the connection is made, both of the variabletransconductance amplifiers GM4 and GM5 see the same reference input[VT+]−[VT−], and since their outputs are differenced, when the gains arematched, the output will be zero. If their gains are not matched, theoutput will be non-zero, driving the differential input to GM6 to avalue such that the gains are matched, with that value being stored oncapacitor CGM. If the closed loop gain of the instrumentation amplifieris sufficiently high in this gain correction configuration, theresulting gain error of the two input stages of the instrumentationamplifier can be made very small. Uncorrected, the mismatch of the twoGM stages GM4 and GM5 will vary significantly over the operatingtemperature range. With gain correction, the mismatch is repeatedlycorrected independent of temperature.

As previously described, the implementation of the auto-zero using fourcapacitors, as shown in FIG. 3, where the input pair of capacitors isused in conjunction with a common input reference, Input VCM Ref, andthe feedback pair of capacitors is used in conjunction with a commonfeedback reference, Feedback VCM Ref, provides a means for decouplingthe common mode voltage of the input pair or feedback pair.

The common mode cancellation voltage of each pair is stored on the CAZcapacitors, in addition to the offset. These stored voltages transferover to FIG. 5 a (combining the auto-zero capability of FIG. 3 with thegain correction of FIG. 4) where this feature allows the two GM stagesto continually operate at a constant and matched common mode condition,independent of the common mode of the differential input (INP−INN),further enhancing the matching of the stages. In a preferred embodiment,Input VCM Ref is taken as one of the inputs INP or INN, filtered througha simple low pass RC filter. Also in a preferred embodiment, FeedbackVCM Ref is taken as one of the feedback signals INP_FB or INN_FB, alsofiltered through a simple low pass RC filter. While these areapproximations, the differential signals will necessarily be relativelylow for instrumentation amplifiers with substantial gain, so that theapproximation will normally be quite good, far better than ignoring theeffect of common mode variation. The choice of input and feedbacksignals preferred may depend on the application. By way of example, ifone input were to operate at a fixed voltage, that input would normallybe the preferred input to use for Input VCM Ref. Similarly, thepreferred bandwidth of the low pass filter may be application dependent.

There is a secondary benefit from gain correction in the combinedcircuit of FIG. 5 a, and that is to improve the accuracy of auto-zerocorrection. In a first-order analysis of auto-zero, there is theassumption that the gains in each half of the instrumentation amplifiermatch, but in fact in a real integrated circuit there is a finite GMmismatch which results in a finite auto-zero error. If a factor for gainmismatch is included in the auto-zero analysis, it can be shown that theGM mismatch degrades the auto-zero correction obtained in the circuit ofFIG. 3. Correspondingly, a residual error in the auto-zero correctionresults in a finite error in the gain correction obtained in the circuitof FIG. 4. When there is a sequence that includes alternating betweenauto-zero and gain correction which is made possible by the circuit ofFIG. 5 a, it can be shown that the residual errors of both will bereduced in steps as an iterative improvement.

The use of auto-zeroing with gain correction as in FIG. 4 is preferablebecause of the complementary effect each has on the other, as explainedabove. However the use of the gain correction without using auto-zeroingas shown in FIG. 4 can also be beneficial, as a significant gaincorrection may be made assuming that the [VT+]−[VT−] differential inputfor the gain correction reasonably dwarfs the difference in inputoffsets Vos of GM4 and GM5.

Aspects of the present invention may be practiced various ways. By wayof example, FIG. 5 a is a Figure incorporating the features of both FIG.3 and FIG. 4 in a single FIGURE. FIG. 5 b is a Figure incorporating thefeatures of both FIG. 3 and FIG. 4 in an instrumentation amplifier withchoppers. FIG. 6 a is a diagram illustrating a feedforwardgain-corrected instrumentation amplifier system using the gaincorrection circuit of FIG. 4. FIG. 6 b is a diagram illustrating again-corrected, auto-zeroed feedforward instrumentation amplifier systemusing the combined auto correction and gain correction configuration ofinstrumentation amplifier of FIG. 5 a. FIG. 7 a is a diagramillustrating a gain-corrected, chopper-stabilized feedforwardinstrumentation amplifier system using the gain correction circuit ofFIG. 4. FIG. 7 b is a diagram illustrating a gain-corrected, auto-zeroedchopper-stabilized feedforward instrumentation amplifier system usingthe combined auto correction and gain correction configuration ofinstrumentation amplifier of FIG. 5 a. FIG. 8 a is a diagramillustrating a gain-corrected, chopper-stabilized feedforwardinstrumentation amplifier system with sample-and-hold using the gaincorrection circuit of FIG. 4, and FIG. 8 b is a diagram illustrating again-corrected, auto-zeroed chopper-stabilized feedforwardinstrumentation amplifier system with sample-and-hold using the combinedauto correction and gain correction configuration of instrumentationamplifier of FIG. 5 a. The outputs of the choppers coupled to the outputof FIGS. 4 (FIG. 8 a) and 5 a (FIG. 8 b) is a square wave of current,which when integrated by the following integrator, provides a triangularwaveform. The sample-and-hold circuit repetitively samples thetriangular waveform at the same place on the waveform (preferably at thecenter). This eliminates the output ripple in VO that may be caused bythe integrator output.

In the foregoing description, the invention has been described withrespect to the use of transconductance amplifiers and the use of MOSdevices. A transconductance amplifier may be referred to as a GM stageor differential amplifier and is considered to be balanced andsymmetrical. The depiction of an inverting or non-inverting outputterminal of a transconductance amplifier is relative to the depiction ofthe inputs of that amplifier. Any systematic depiction of input andoutput polarities and the resulting connections which meet therequirements of negative feedback to achieve stability are also includedin this disclosure. Such a construction of the present invention ispreferred, but is not a limitation of the invention. Other devices, suchas junction transistors, may also be used if desired. Also, referencesherein and in the following claims to an amplifier or a transconductanceamplifier do not suggest only single stage amplifiers, but may alsoinclude cascaded amplifier stages. Assuming multiple stages in the uppersignal flow path of FIGS. 6 a through 8 b, the offset correction fromthe integrator may be applied to any of the multiple stages. Similarly areference to an amplifier or transconductance amplifier having twodifferential inputs generally includes amplifiers and transconductanceamplifiers having parallel differential input stages in which theoutputs of the input stages are combined to provide a single paralleloutput as the amplifier output or for coupling to a subsequentdifferential stage of the amplifier. Generally, the output of adifferential amplifier requires additional circuitry to regulate thecommon mode voltage of the differential outputs relative to a fixedpotential within the circuit, as is well known in the art. Thisrequirement applies as appropriate to the differential amplifiersincorporated in the preferred embodiments of the present invention. Theaddition or removal of series and redundant switches is covered by theclaims of this invention. Finally, the invention may be used alone, insub-combinations or with other instrumentation amplifier performanceenhancing embellishments, as desired.

Thus while certain preferred embodiments of the present invention havebeen disclosed and described herein for purposes of illustration and notfor purposes of limitation, it will be understood by those skilled inthe art that various changes in form and detail may be made thereinwithout departing from the spirit and scope of the invention.

1. An instrumentation amplifier having first and second differentialinstrumentation amplifier inputs and a differential instrumentationamplifier output comprising: first and second variable gain differentialinput amplifiers, each having a differential input, and having theirdifferential outputs coupled to provide a common differential output; athird differential amplifier having a first capacitor coupled across itsdifferential input, and having each of its differential outputs coupledto control the gain of a respective one of the first and second variablegain differential input amplifiers; first switches coupled to the commondifferential output to disconnect the common differential output of thefirst and second variable gain differential input amplifiers from thedifferential instrumentation amplifier output and couple the output ofthe first and second variable gain differential input amplifiers to thedifferential input of the third differential amplifier; second switchesoperative with the first switches to disconnect the first and seconddifferential instrumentation amplifier inputs from the first and secondvariable gain differential input amplifier differential inputs, and tocouple the differential inputs of the first and second variable gaindifferential input amplifiers to a single differential voltage with apolarity whereby the combined output is the difference in differentialoutput of the first and second variable gain differential inputamplifiers; whereby on momentarily operating the first and secondswitches, a voltage is stored on the first capacitor providing adifferential output of the third differential amplifier correcting thedifferences in gain in the first and second variable gain differentialinput amplifiers.
 2. The instrumentation amplifier of claim 1 furthercomprising a first chopper coupled to the first differentialinstrumentation amplifier input, a second chopper coupled to the seconddifferential instrumentation amplifier input and a third chopper coupledto the differential instrumentation amplifier output.
 3. Theinstrumentation amplifier of claim 1 wherein the second switches arecoupled by second capacitors to the differential inputs of the first andsecond variable gain differential input amplifiers, and furthercomprising third switches for disconnecting the first and seconddifferential instrumentation amplifier inputs and coupling the commondifferential output to the differential inputs of the first and secondvariable gain differential input amplifiers, and for coupling the inputsto the capacitors coupled to the differential input of the firstvariable gain differential input amplifier together and for coupling theinputs to the capacitors coupled to the differential input of the secondvariable gain differential input amplifier together; whereby onmomentarily closing the third switches, a voltage is stored on thesecond capacitors, zeroing the first and second variable gaindifferential input amplifiers.
 4. The instrumentation amplifier of claim3 further comprising a first chopper coupled to the first differentialinstrumentation amplifier input, a second chopper coupled to the seconddifferential instrumentation amplifier input and a third chopper coupledto the differential instrumentation amplifier output.
 5. Theinstrumentation amplifier of claim 4 wherein the third switches alsocouple the inputs to the capacitors coupled to the differential input ofthe first variable gain differential input amplifier to a common modereference, and couple the inputs to the capacitors coupled to thedifferential input of the second variable gain differential inputamplifier to another common mode reference.
 6. The instrumentationamplifier of claim 5 further comprising a first chopper coupled to thefirst differential instrumentation amplifier input, a second choppercoupled to the second differential instrumentation amplifier input and athird chopper coupled to the differential instrumentation amplifieroutput.
 7. A feedforward instrumentation amplifier system having adifferential instrumentation amplifier system input and aninstrumentation amplifier system output comprising: an instrumentationamplifier having a first and second differential instrumentationamplifier inputs and a differential instrumentation amplifier output,and having: first and second variable gain differential inputamplifiers, each having a differential input, and having theirdifferential outputs coupled to provide a common differential output; athird differential amplifier having a first capacitor coupled across itsdifferential input, and having each of its differential outputs coupledto control the gain of a respective one of the first and second variablegain differential input amplifiers; first switches coupled to the commondifferential output to disconnect the common differential output of thefirst and second variable gain differential input amplifiers from thedifferential instrumentation amplifier output and couple the output ofthe first and second variable gain differential input amplifiers to thedifferential input of the third differential amplifier; second switchesoperative with the first switches to disconnect the first and seconddifferential instrumentation amplifier inputs from the first and secondvariable gain differential input amplifier differential inputs, and tocouple the differential inputs of the first and second variable gaindifferential input amplifiers to a single differential voltage with apolarity whereby the combined output is the difference in differentialoutput of the first and second variable gain differential inputamplifiers; whereby on momentarily operating the first and secondswitches, a voltage is stored on the first capacitor providing adifferential output of the third differential amplifier correcting thedifferences in gain in the first and second variable gain differentialinput amplifiers; fourth, fifth and sixth differential input amplifiers,the fourth and fifth amplifiers each having a differential output; thedifferential outputs of the fourth and fifth differential inputamplifiers being coupled together and to the differential input of thesixth differential input amplifier, the output of the sixth differentialinput amplifier being coupled to the instrumentation amplifier systemoutput; the differential instrumentation amplifier system input beingcoupled to the differential input of the fourth differential inputamplifier and to the differential input of the first variable gaindifferential input amplifier; the differential input of the fifthdifferential input amplifier and the differential input of the secondvariable gain differential input amplifier being coupled together forcoupling to a differential feedback signal from the instrumentationamplifier system output; the common differential output of the first andsecond variable gain differential input amplifiers being coupled to anintegrator, an output of the integrator being coupled to thedifferential input of the sixth differential input amplifier.
 8. Thefeedforward instrumentation amplifier system of claim 7 wherein one ofthe differential feedback signals is coupled to a reference voltage. 9.The feedforward instrumentation amplifier system of claim 7 wherein theinstrumentation amplifier further comprises a first chopper coupled tothe first differential instrumentation amplifier input, a second choppercoupled to the second differential instrumentation amplifier input and athird chopper coupled between the differential instrumentation amplifieroutput and the integrator.
 10. The feedforward instrumentation amplifiersystem of claim 9 further comprised of a sample and hold circuit betweenthe output of the integrator and the differential input of the sixthdifferential input amplifier.
 11. The feedforward instrumentationamplifier system of claim 7 wherein in the instrumentation amplifier,the second switches are coupled by second capacitors to the differentialinputs of the first and second variable gain differential inputamplifiers, and further comprising third switches for disconnecting thefirst and second differential instrumentation amplifier inputs andcoupling the common differential output to the differential inputs ofthe first and second variable gain differential input amplifiers, andfor coupling the inputs to the capacitors coupled to the differentialinput of the first variable gain differential input amplifier togetherand for coupling the inputs to the capacitors coupled to thedifferential input of the second variable gain differential inputamplifier together; whereby on momentarily closing the third switches, avoltage is stored on the second capacitors, zeroing the first and secondvariable gain differential input amplifiers.
 12. The feedforwardinstrumentation amplifier system of claim 11 wherein the instrumentationamplifier further comprises a first chopper coupled to the firstdifferential instrumentation amplifier input, a second chopper coupledto the second differential instrumentation amplifier input and a thirdchopper coupled to the differential instrumentation amplifier output.13. The feedforward instrumentation amplifier system of claim 11 whereinin the instrumentation amplifier, the third switches also couple theinputs to the capacitors coupled to the differential input of the firstvariable gain differential input amplifier to common mode reference, andcouple the inputs to the capacitors coupled to the differential input ofthe second variable gain differential input amplifier to another commonmode reference.
 14. The feedforward instrumentation amplifier system ofclaim 13 wherein in the instrumentation amplifier further comprises afirst chopper coupled to the first differential instrumentationamplifier input, a second chopper coupled to the second differentialinstrumentation amplifier input and a third chopper coupled to thedifferential instrumentation amplifier output.
 15. The feedforwardinstrumentation amplifier system of claim 14 further comprised of asample and hold circuit between the output of the integrator and thedifferential input of the sixth differential input amplifier.
 16. In anamplifier unit having first and second variable gain differential inputamplifiers, and having first and second amplifier unit differentialinputs and an amplifier unit differential output, a method of gaincorrection to match the gains of the first and second differential inputamplifiers comprising: coupling differential outputs of the first andsecond variable gain differential input amplifiers in common; couplingthe first and second amplifier unit differential inputs to thedifferential inputs of the first and second variable gain differentialinput amplifiers, respectively; coupling the common coupling of thedifferential outputs of the first and second variable gain differentialinput amplifiers to the amplifier unit differential output; providing athird differential input amplifier having a respective one of itsdifferential output to control the gain of a respective one of the firstand second variable gain differential input amplifiers; and,repetitively; momentarily decoupling the first and second amplifier unitdifferential inputs from the differential inputs of the first and secondvariable gain differential input amplifiers, respectively, andmomentarily de-coupling the common coupling of the differential outputsof the first and second variable gain differential input amplifiers fromthe amplifier unit differential output, and momentarily coupling each ofthe common connections of the first and second variable gaindifferential input amplifiers to a respective one of the differentialinputs of the third differential input amplifier, and momentarilyapplying a differential input to the first and second variable gaindifferential input amplifiers, and capacitively storing a differentialvoltage on the differential input of the third differential inputamplifier.
 17. The method of claim 16 wherein the differential voltageon the differential input of the third differential input amplifier isstored on a capacitor coupled across the differential input of the thirddifferential input amplifier.
 18. The method of claim 16 wherein theamplifier unit is in a feedforward signal path of a feedforwardgain-corrected instrumentation amplifier system.
 19. The method of claim18 further comprising: chopping the first and second amplifier unitdifferential inputs and the amplifier unit differential output.
 20. Themethod of claim 19 further comprising: integrating the chopped amplifierunit differential output; sampling the integrated chopped amplifier unitdifferential output by a sample and hold circuit; and coupling an outputof the sample and hold circuit to an output of the feedforwardgain-corrected instrumentation amplifier system.
 21. The method of claim16 wherein the first and second variable gain differential inputamplifiers have capacitively coupled differential inputs, and furthercomprising: auto-zeroing the first and second variable gain differentialinput amplifiers by: momentarily de-coupling the first and secondamplifier unit differential inputs from the capacitively coupleddifferential inputs of the first and second variable gain differentialinput amplifiers, respectively, and momentarily de-coupling the commoncoupling of the differential outputs of the first and second variablegain differential input amplifiers from the amplifier unit differentialoutput; and momentarily shorting the capacitive coupled differentialinputs of each of the first and second capacitively coupled differentialinput amplifiers; thereby canceling input offsets by voltages stored onthe capacitive coupling on the differential inputs of the first andsecond differential input amplifiers.
 22. The method of claim 21 furthercomprising: when momentarily shorting the capacitive coupleddifferential inputs of each of the first and second capacitively coupleddifferential input amplifiers, coupling the shorted differential inputto the first differential input amplifier to a voltage approximating acommon mode voltage of the first amplifier unit differential input, andcoupling the shorted differential input to the second differential inputamplifier to a voltage approximating a common mode voltage of the secondamplifier unit differential input.
 23. The method of claim 21 furthercomprising: alternately correcting the gain of the first and seconddifferential input amplifiers and auto-zeroing the first and secondvariable gain differential input amplifiers.
 24. The method of claim 21further comprising: chopping the first and second amplifier unitdifferential inputs and the amplifier unit differential output.
 25. Themethod of claim 21 wherein the amplifier unit is in a feedforward signalpath of a feedforward gain-corrected instrumentation amplifier system.26. The method of claim 25 further comprising: chopping the first andsecond amplifier unit differential inputs and the amplifier unitdifferential output.
 27. The method of claim 26 further comprising:integrating the chopped amplifier unit output; sampling the integratedchopped amplifier unit differential output by a sample and hold circuit;and coupling an output of the sample and hold circuit to an output ofthe feedforward gain-corrected instrumentation amplifier system.